1. Field of the Invention
The present invention relates to a lock-up type automatic transmission and more particularly to a lock-up control system thereof.
2. Description of the Prior Art
Most automatic transmissions include a torque converter in a power transmission system thereof to increase the torque fed to the transmission from an engine. The common torque converters are of the type wherein a pump impeller driven by an engine rotates an operating oil within the torque converter so as to cause the rotation of the operating oil to rotate a turbine runner under the reaction of a stator to increase the torque (torque-converter mode). During the operation of the torque converter, slip between the pump impeller and turbine runner is unavoidable and thus the automatic transmission including such a torque converter in its power transmission system has the advantage of easy operation but a drawback in that the power transmission effeciency is poor and thus the fuel economy is bad. To alleviate this drawback, there has been proposed a so-called torque converter with a direct clutch (also called a lock-up torque converter) wherein during relatively high vehicle speed operation when the torque variation of an engine does not create a problem, the turbine runner is directly connected to the pump impeller (lock-up mode), thereby eliminating slip. Some vehicles use a lock-up type automatic transmission including a torque converter of this kind in the transmission system.
However, with conventional lock-up type automatic transmissions, when in each of the gear ratios the vehicle speed exceeds a predetermined value (lock-up vehicle speed value), the torque converter with the direct clutch is shifted into a lock-up mode, or when the vehicle speed exceeds a predetermined value (lock-up vehicle speed value) in a single predetermined gear ratio, the torque converter with a direct clutch is shifted into a lock-up mode. In both of these cases, the lock-up vehicle speed values are fixed and are constant. Usually, such lock-up vehicle speed values are set to the appropriate values suitable for optimum operation after the warming up of the engine.
However, during engine operation when the warming up operation has not been completed, the combustion within the engine is not stable and thus the engine operation under this mode is equivalent to the engine operation with an excessively large load, so that if the torque converter with a direct clutch is shifted into a lock-up mode when the vehicle speed is above any one of the lock-up vehicle speed values, not only does the driveability worsen due to the shortage of torque, but also the unburnt components within the exhaust gas increases.
Furthermore, even after warming up, engine operation at large load or small load is sacrificed, if as in the conventional case the lock-up vehicle speed value is constant, because the vehicle speed value may not be suitable for engine operation of both high and low load conditions. That is, where the lock-up vehicle speed value is set at a value suitable for the case when the engine load is large, the lock-up vehicle speed value becomes unncessarily high for the case when the engine load is small, thus failing to accomplish an objective of the lock-up type automatic transmission of improving fuel economy. Where the lock-up vehicle speed value is set at a value suitable for the case when the engine load is small, the lock-up vehicle speed value is too low for the case when the engine load is large, worsening the driveability because of the shortage of a torque and at the same time increasing the unburnt components within the exhaust gases.